I am trying to get a spinning stones from the Bahro cave that you link to from En'Cana having touched all the shell cloths. There are 2 one which you can get by pressing the shell symbol in the cave on the side of the 'hole' you appear on and one, which I guess you collect by pressing the shell on the other side of the hole.

Thing is I can't get to this second shell. I have tried various jumps but the last one had me fall straight through the ledge on the other side of the hole.

This suggests I need to do something to extend the other platform, but I have no idea what.

If you have not completed En'cana please don't read the spoiler, for those who have, any ideas what I am doing wrong?